RELATIONSHIP BETWEEN RAT SPINAL SUBSTANCE-P AND THE ELEVATION OF PAIN THRESHOLD INDUCED BY THIAMINE-DEFICIENCY

Thiamine deficiency (TD) in rats causes elevation of the pain threshold in correlation with the degree of TD. To clarify correlation between change in substance P (SP) content in the rat spinal cord and increased pain threshold after starting TD treatment, immunohistochem-ical assay of spinal SP was performed by determining fluorescence intensity with two- or three-dimensional microphotometry. Fluorescent intensity of SP was high in laminae I and II of the ventral part and central duct of the spinal cord in normal rats. The intensity of the SP fluorescence in the ventral part decreased significantly in the 25 days after starting TD treatment. This was concomitant with increased pain threshold judged by both the hot-plate and tail-flick tests. Treatment with thiamine HCl on day 14 of the TD treatment completely restored both effects to normal control levels, but thiamine HCl treatment after possible degeneration of the afferent nerve fibers failed to reverse the effects. The results indicate a possibly great contribution of SP in the ventral part of the spinal cord to elevation of the pain threshold by TD.